Meet the Voracious Rodents Munching Through California's Wetlands

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  • Joe Eaton is a Berkeley-based science and environmental writer and a frequent contributor to Estuary News

California has its hands full with wildfires, toxic cultural schisms, and an unfriendly presidency. We didn’t need an outbreak of large voracious rodents, munching their way through the wetlands and tunneling into flood-control levees. But they’re here, or, more accurately, back. Meet the nutria.

Rodent Stories

These rat-like creatures, native to subtropical and temperate South America and featured in the new documentary film “Rodents of Unusual Size” (a nod to “The Princess Bride”), aren’t the world’s largest rodents. That would be the capybara, a 150-pound amphibious guinea pig. But the 20-pound nutria, chunky animals with long hairless tails and alarming orange incisor teeth, are big enough.

Also called coypu, they were introduced to the Louisiana bayous, the Chesapeake Bay marshes, and every continent except Australia and Antarctica (plus New Zealand and Japan) by fur farmers. That’s how they got to California where, according to the U.S. Geological Survey, the federal agency that deals with aquatic nuisance animals, they were first recorded in Santa Clara County in 1899. Introductions peaked during the Great Depression.

As with other exotics like bullfrogs and red foxes, enough nutria escaped from captivity or were liberated when the market went bust to establish a foothold in California’s wilds. Their potential impact was significant. Nutria can put away a quarter of their weight in vegetation — leaves, roots, and all — in a day. They eat any kind of plant that grows in or near water, such as rice and sugar cane, and wild plants — tules, cattails, etc. — which are important to marsh ecosystems. Sloppy eaters, they trash 10 times more plant material than they consume, defoliating acres of marsh and exacerbating erosion. Their burrows, up to 20 feet deep and 164 long, turn levees and roadbeds into Swiss cheese. Nutria-borne pathogens include tapeworms, liver flukes, and a nematode worm that causes “swimmer’s itch.” Unsurprisingly, they were targeted by wildlife agencies and believed eradicated by the 1970s.

Nutria Timo Sack / WikiCommons / CC-BY-SA-2.5

Last year, though, a trapper with the U.S. Department of Agriculture’s Wildlife Services, whose mission is removing unwanted wildlife, found an unfamiliar creature in a beaver trap at a private duck-hunting club in Merced County: a pregnant nutria. Had they been hiding out in the tules, unrecognized or mistaken for muskrats or beavers, or did someone illegally import them from out of state and release them in the San Joaquin Valley? “It’s hard to conceive nutria persisted in the state at low numbers and have gone undetected for almost 50 years, so we believe it was an introduction,” says Martha Volkoff of the California Department of Fish and Wildlife’s Invasive Species Program. 

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“The optimistic view was that this was a very isolated population,” Volkoff recalls. “Perhaps if we acted quickly we could remove them all.” But it wasn’t going to be that easy. Deploying traps and cameras, Fish and Wildlife found more nutria, from Fresno County to San Joaquin County. They had to interrupt their efforts during waterfowl season and pick them up again in the spring. As of mid-September 280 had been captured. “That’s not counting the ones that came in as roadkill or were taken by farmers or killed by dogs,” Volkoff adds. One found under a parked car was netted and released by an animal control officer who didn’t realize what it was.

Trailer for film "Rodents of Unusual Size"

Volkoff explains that the nutria infestation has been designated an incident, enabling the state agency to redirect staff and funds. They’re trying to persuade landowners, particularly in the Delta, to give surveyors access to their properties. Wildlife Services is helping trap and remove the rodents. There are encouraging precedents for eradication: Chesapeake Bay is now nutria-free. But their tribble-like fecundity will be a challenge. Females can give birth at the age of 8 months and produce three litters of up to 13 young in a year. 

An appetite for the invasive water hyacinth may be the nutria’s main redeeming trait, but it’s not enough to mitigate the havoc they wreak. California’s sizable investment in restoring freshwater Delta marshes raises the stakes. 

Eradication campaigns in Louisiana and elsewhere have promoted nutria as food. Celebrity Cajun chefs have enlisted in what journalist Calvin Trillin called “an attempt to do to the nutria on purpose what Paul Prudhomme had done to the redfish by accident.” A Maryland state biologist described the meat as “a lot better than muskrat, not nearly as greasy.” As part of their Kickstarter campaign, the makers of “Rodents of Unusual Size” taste-tested several nutria products and preparations, giving the jambalaya a thumbs-up but calling the sausage distinctly swampy in flavor. Even California’s adventurous foodies may not be ready for this one. 

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  • About the Author

    Joe Eaton

    Joe Eaton is a Berkeley-based science and environmental writer and a frequent contributor to Estuary News

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